The lens in the eye is built up in layers, like
an onion. New layers are constantly being added to the outside,
compressing and hardening those in the centre. This results in a
gradual stiffening of the lens over time. The ciliary muscle grows
stronger to compensate for this, doubling in size between the ages
of 20 and 40. Unfortunately, as we do our close work in relatively
poor light with enlarged pupils, the eye is unable to focus as well
as it might and the ciliary muscle does not get as much exercise
as it needs. The growth of the ciliary muscle falls behind what
is required for the closest focusing. This ‘near point’ gradually
moves further away until by the mid-40s it starts to interfere with
reading small print. This is called presbyopia. The conventional
treatment for presbyopia is to place a lens in front of the eye
to add the extra focusing power that the eye is unable to provide.
This immediately brings print into focus but also leaves the ciliary
muscle less work to do. With habitual use of reading glasses the
ciliary muscle gets less exercise and starts to get weaker, further
reducing the ability to focus close up.